Cellulose acetate, particularly cellulose triacetate, which has excellent physical properties, particularly good processability and high optical properties, has been utilized through many years in the field of plastics, fibers, films (e.g., photographic film, etc.). Further, cellulose acetate has attracted attention from the viewpoint of global environment, because it possesses biodegradability.
In general, cellulose acetate is a semi-synthetic polymer obtained by esterification from cellulose as a starting material using acetic anhydride. Presently commercially available cellulose acetates may be roughly divided into two groups mainly according to degree of acetylation thereof. One is cellulose triacetate (hereinafter referred to as CTA) having degree of acetylation of not less than 59%. The other is cellulose diacetate of wide range. Among them those having degree of acetylation of about 50 to 59% are referred to as cellulose diacetate (CDA). In other words, it is cellulose acetate solubLe in acetone.
As for use of cellulose acetate, in particular, CTA includes, it can be used as various films including base materials for photographic film as well as fibers and medical materials. In general, molded product of cellulose acetate is rigid and brittle. Such properties may become more significant the higher acetylation degree becomes. Physical properties of polymeric materials greatly depend on their crystallizability. That is, those having high crystallizability are imparted with strength while flexibility, for example, elongation is reduced, resulting in brittle product. Of course, CTA is not an exception, and has high crystallizability due to the homogeneity of its structure. That is, in cellulose acetate, higher degree of acetylation results in higher crystallizability. In general, lower molecular weight material becomes a nucleus, forming a crystal. Accordingly, when CTA, CDA are used, a plasticizer is generally added to impart flexibility to the molded product. For example, acetate plastics used for a grip of a screw-driver and the like often utilizes phthalate plasticizer such as diethyl phthalate. Cellulose acetate, particularly, CTA, is used as raw materials for various films due to its excellent transparency, while it has defects, for example, providing rigid and brittle film. To overcome its physical defects, a plasticizer is also used in this case. Addition of a component such as a plasticizer is accompanied not only with poor yield of the final product due to bleedout during molding as well as with economical disadvantage. Thus, cellulose acetate with excellent physical properties as well as characteristics of CTA has been expected.
On the other hand, as increase of the uses of cellulose acetate, speeding up of processing technique is required, and high speed molding, high speed spinning, high speed processing of the molded product have been tried. For example, in the production of film, it is proposed to cast a solution of cellulose acetate at a high speed to mold into film. In order to improve moldability corresponding to such speeding up, it is proposed that average degree of polymerization of cellulose acetate is reduced to lower solution viscosity of cellulose acetate. However, using cellulose acetate with low degree of polymerization, physical strength of the molded product is impaired.
Among cellulose acetates, cellulose triacetate which has been used in various purposes, is crystalline and its solubility in a solvent is reduced as crystallization properties increase. To improve solubility in a solvent and moldability of cellulose triacetate, it is considered to be useful to make amorphous or lower crystallizability by reducing degree of substitution of cellulose triacetate. However, when degree of substitution of cellulose triacetate is reduced, hygroscopicity is increased and dimensional accuracy of the molded product is impaired.
Therefore, it is difficult to improve solubility in a solvent, moisture resistance, moldability and the like by reducing crystallizability of cellulose acetate.
Accordingly, an object of the present invention is to further improve characteristics of CTA and improve physical properties of the molded product, and to provide a process for production of such cellulose acetate.
An another object of the present invention is to provide cellulose acetate with low crystallizability and excellent moldability.
Still another object of the present invention is to provide cellulose acetate with high solubility in a solvent and moldability, in spite of high average degree of polymerization and high average degree of substitution.